1. Field of the Invention
This invention relates to a communication system and a communication method for carrying out data communications among a plurality of communication stations, and in a communication apparatus used in these communication system and communication method, this invention particularly relates to a communication system, a communication method, and a communication apparatus suitable for applying to wireless LAN (Local Area Network).
2. Description of the Related Art
Recently, for example, the near filed wireless communication techniques have been developed for wirelessly coupling various types of data processing terminals together, such as personal computers or portable data terminals, and their peripheral devices. Typically, the wireless LAN of the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard is now widely used.
In the wireless LAN according to the IEEE 802.11 standard, as techniques for the media access control (MAC) system regarding the protocol for distributed control, the centralized control or the like in data link layer, the contention free interval for the media access control by polling and the contention interval for the media access control by the carrier sensing method are standardized. Out of them, the contention period for the carrier sensing method is widely used.
More specifically, as the contention period, the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) method following the autonomous distributed control method used in Ethernet™ has been standardized. This CSMA/CA method is generally defined as a technique to avoid a collision of data from a communication station trying a data transmission with data being transmitted by another communication station, wherein the communication station confirms the used condition of the wireless channel by previously doing the carrier sensing method. In this case, if the band is not used, the communication station transmits the data, but if the band is used, the communication station postpones the transmission of the data until the band becomes an idle state. In the wireless LAN according to the IEEE 802.11 standard, an access point AP provided as a control station and also a plurality of stations STA existing within the radio wave accessible area of the access point communicate with each other by carrying out the procedures according to this CSMA/CA method.
As a technique regarding the CSMA/CA method, for example, Japanese Laid Open Patent application No. 2002-217914 discloses such a technique. That is, this patent document discloses a technique to improve the directivity gain of an antenna apparatus used in the CSMA/CA method to increase the communication quality.
In the media access control method according to the CSMA/CA method, there is an essentially unavoidable problem because this method assumes that a plurality of communication stations can detect wireless signals each other. That is, there is so-called hidden station problem. To solve this hidden station problem, the control using the so-called RTS (Request To Send) and CTS (Clear To Send) signals were developed in the wireless LAN according to IEEE 802.11 standard.
To explain the control with the RTS signal and the CTS signal, it is considered that an access point AP becomes a source, and tries to transmit data to a station STA. In this case, according to the wireless LAN of the IEEE 802.11, the access point AP that carries out the carrier sense in advance transmits the RTS signal as shown in FIG. 14, and after a predetermined interval SIFS (Short Inter Frame Space), the station STA that receives this RTS signal as a destination transmits the CTS signal as a response. The access point AP that receives the CTS signal starts transmission of data (Fragment) after the predetermined interval SIFS, and after the predetermined interval SIFS since the completion of the transmission of the data, the station STA that receives the data replies the so-called ACK (Acknowledgment) signal.
In this condition, other stations STA that do not communicate know that the channel is occupied for a predetermined interval in accordance with the exchanges of the RTS signal, the CTS signal, the data, and the ACK signal. Accordingly, they set the intervals as standby intervals in counter values NAV (RTS), NAV (CTS), NAV (Fragment), and NAV (ACK), which are called NAV (Network Allocation Vector), to record transmission operations or the like. In the wireless LAN according to the IEEE 802.11 standard, upon the elapse of this standby interval, the return of the ACK signal from the station STA as a destination to the access point AP also terminates. Further, in the wireless LAN according to the IEEE 802.11 standard, after a predetermined interval DIFS (Distributed Inter Frame Space) elapses from when the transmission of the ACK signal is terminated, decrements are started in the counters in which values are determined with uniform random numbers from zero to CW (Contention Window) for back off provided to avoid data collision, respectively. Then, one of stations STA and the access point AP of which counter value reaches zero first transmits the RTS signal to occupy the channel at the next interval.
As mentioned above, the wireless LAN according to the IEEE 802.11 standard is directed to solve the hidden station problem by avoiding data collisions in accordance with the notification of using the channel to other communication stations with the RTS/CTS control.
Here, the IEEE 802.11a standard is provided as one of standards for the physical layer operating in the MCA layer defined by the IEEE 802.11 standard. In the wireless LAN, the use of this physical layer provides wireless communications at a transmission rate of about 50 Mbps at its maximum. Actually, while the transmission rate decreases in the wireless LAN according to the propagation circumstance or the like, it is supposed that the transmission rate can be obtained around a half of the above mentioned maximum transmission rate.
However, for example, if it is assumed that a large capacity of data is transmitted as in the circumstance that this wireless LAN is established in a house, and a plurality of streams are transmitted to the television sets located at a plurality of rooms from a predetermined server, the above-described data transmission capacity may be insufficient. Thus, there is a demand for increasing a capacity of communication.
Adaptive array antennas include a plurality of antenna elements each having the same characteristic, are able to control amplitude and phase of oscillation in each antenna element independently, and provide communications with a plurality of stations on the same frequency at the same time. Further, the adaptive array antenna can reduce the possibility of interference among a plurality of radio waves. Because of this, the adaptive antenna array attracts attention as a technique providing an improvement in the frequency utilizing efficiency.
For example, Japanese Laid-open patent application Nos. 2002-51375 and 2001-339331 disclose the adaptive array antennas. The Japanese Laid-open patent application No. 2002-51375 discloses a technique providing an efficient downlink high speed packet transmission with the adaptive array antenna, and the Japanese Laid-open patent application No. 2001-339331 discloses a technique for communication with an optimum directivity.